Light olefin products (e.g. ethylene, propylene, and butene) may be generated by various technologies, such as gas to olefins, methanol to olefins, steam cracking, pyrolysis or fluid catalytic cracking. These products contain highly unsaturated byproducts, such as alkynes and alkadienes. These byproducts are subsequently removed from light olefins because they can be poisons to downstream processes, such as olefin polymerization catalysts.
One process for removing unsaturated byproducts, such as alkynes and alkadienes, from light olefin streams is selective hydrogenation. Alkynes include acetylene and/or methyl acetylene, while alkadienes include propadiene and/or butadiene. The selective hydrogenation has been carried out using a variety of catalysts. Examples of selective hydrogenation catalysts (e.g., catalytic particles) include nickel or palladium and mixtures thereof supported on alumina.
To perform the selective hydrogenation, four unit types are typically used: (i) front-end selective catalytic hydrogenation converters, (ii) back-end selective catalytic hydrogenation converters, (iii) methyl acetylene/propadiene (MAPD) selective catalytic hydrogenation converters and (iv) butadiene (BD) selective catalytic hydrogenation converters. These converters typically involve different feeds based on the specific process.
Typical acetylene conversion processes utilize fixed bed tubular converters incorporating engineered catalyst structures to manage heat and mass transfer within the converter. The engineered catalyst particles may be impregnated or coated with active catalyst to convert feeds (e.g., acetylene) into products (e.g., ethylene). These processes are generally utilized with lower temperature pyrolysis processes, such as steam cracking, which produce ethylene along with other lower amounts of byproducts, such as acetylene. As an example, the acetylene processed in a steam cracking process is typically less than (<) 2 mole percent (mol %) on feed.
With higher acetylene concentrations, U.S. Pat. No. 4,705,906 describes a process that utilizes greater than (>) 1 mol % carbon monoxide in its process. The catalyst comprises a metal oxide, sulfide, mixture of metal oxides, or sulfides having hydrogenation activity, for example ZnO either alone or in combination with other metal oxides or sulfides. As other examples, U.S. Pat. No. 7,153,807 discloses a selective hydrogenation process that uses non-palladium catalyst as the selective hydrogenation catalyst, while U.S. Pat. No. 7,404,936 discloses the use of microchannel converters.
However, these processes suffer from several limitations. For instance, as the process involves exothermic reactions, the process may lose control of the reactions if the temperature within the unit is not properly managed. For streams with low levels of acetylene (e.g., <2 mol %), the reactions may be managed selectively using conventional techniques because of the lower catalyst activity or heat release rates. However, for streams containing higher levels of acetylene (e.g., ≧2 mol %), conventional processes have problems controlling the reaction temperatures and still remaining highly selective. In addition, the conventional processes are limited by heat and/or mass transfer, as only a small part of the converter volume is used by the active catalyst, and the catalyst has to be configured with low metal loadings and catalytic activity. That is, as the process does not efficiently remove heat, the process has to limit reactions to prevent overheating of the unit. As such, the conventional processes are limited by heat generation and fail to effectively recover energy released in the process.
Further, the production of significant amounts of undesirable compounds, such as saturates (e.g., ethane, propane, butane), as well as the production of green oil (C4+ oligomer compounds), are problematic with the higher acetylene concentration containing feeds. These saturates are typically formed due to over-hydrogenation of the alkynes and/or alkadienes and the non-selective hydrogenation of olefins. Additionally, green oil is generally formed as a result of oligomerization of the alkynes and/or alkadienes and/or olefins. Both saturates and green-oil are undesirable due to a loss of the desired mono-olefins component of the product stream along with incremental hydrogen consumption. Green oil is additionally troublesome in that it further decreases catalyst life by depositing heavy compounds on the catalyst surface.
As yet another problem, the selectivity is typically modest for vapor phase processes with a portion of the acetylene and/or ethylene converting to ethane and/or other undesired products. This low selectivity may not be problematic for lower temperature conversion processes (e.g., steam cracking), which involves streams having a relatively low acetylene content. However, for higher acetylene content streams, the lower selectivity results in recycles and/or multiple conversion stages. These inefficiencies increase cost of equipment and operations and add unnecessary complexity to the system. To address this concern, some processes may involve absorption to enhance selectivity, such as U.S. Pat. No. 7,692,051. While these processes may enhance the selectivity, they tend to be less energy efficient.
Accordingly, enhancements in selective hydrogenation processes are desired to increase the hydrogenation of alkynyl-containing compounds and/or polyunsaturated compounds over hydrogenation of mono-unsaturated compounds. Additional enhancements in selective hydrogenation processes are also desired, such as increasing heat recovery of the reaction process and increasing feed conversion rate relative to converter volume.